On Sun, Aug 3, 2008 at 7:36 PM, Rene Herman <rene.herman@xxxxxxxxxxxx> wrote:
> On 03-08-08 09:58, Manish Katiyar wrote:
>
>> ok... I know i am going to be embarrassed but I am confused by the below
>> function.
>
> I'd not be embarrased...
>
>> The last parameter in do_mpage_readpage() is of type get_block_t, but
>> when passed to block_read_full_page() it gets passed as get_block_t *
>
> The exact nature of get_block_t is important here. Variables of type
> get_block_t are function pointers -- have function type:
>
> typedef int (get_block_t)(...);
>
>> static struct bio * do_mpage_readpage(............., get_block_t
>> get_block) {
>> ................
>> ................
>> if (page_has_buffers(page))
>> goto confused;
>> .................
>> confused:
>> if (!PageUptodate(page))
>> block_read_full_page(page, get_block);
>> .........
>> }
>>
>> int block_read_full_page(struct page *page, get_block_t *get_block) {
>> .........
>> ..........
>> }
>>
>>
>> How does this work ? :-( ....... Isn't gcc supposed to catch this and
>> flag warning unless I am missing something very obvious.
>
> In C, except as an operand of sizeof or unary &, an expression having
> function type is automatically converted from function type to pointer to
> function.
>
> Or put differently: except for sizeof and unary &, "function_designator" and
> "&function_designator" are interchangeable as expressions. You can therefore
> write the call as:
>
> block_read_full_page(page, &get_block);
Thanks a lot Rene :-) ...... Yeah I was expecting this declaration.
That clears my doubt.
Thanks -
Manish
>
> without changing anything, after which I suppose you'd be fine with the
> declarations.
>
> Now, granted, this specific example is still not nice. I haven't checked
> other uses but it appears that block_read_full_page() should've been
> declared not with a get_block_t *, but simply with a get_block_t. If you can
> convince yourself of such, you could submit it...
>
> Rene.
>
>
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